1. Field of Invention
The present invention relates generally to equipment used in semiconductor processing. More particularly, the present invention relates to a mechanism which is arranged to reduce the amount of particle contamination on a reticle used in an extreme ultraviolet lithography system.
2. Description of the Related Art
In photolithography systems, the accuracy with which patterns on a reticle are projected off of or, in the case of extreme ultraviolet (EUV) lithography, reflected off of the reticle onto a wafer surface is critical. When a pattern is distorted, as for example due to particle contamination on a surface of a reticle, a lithography process which utilizes the reticle may be compromised. Hence, the reduction of particle contamination on the surface of a reticle is crucial.
Photolithography systems typically use pellicles to protect reticles from particles. As will be appreciated by those skilled in the art, a pellicle is a thin film on a frame which covers the patterned surface of the reticle to prevent particles from becoming attached to the patterned surface. Pellicles, however, are not used to protect EUV reticles, since thin films generally are not suitable for providing protection in the presence of EUV radiation. Principles of thermophoresis may also be applied to protect reticles from particle contamination by maintaining reticles at a higher temperature than their surroundings, and, therefore, causing the particles to move from the hotter reticle to the cooler surroundings, e.g., cooler surfaces.
Since thermophoresis generally may not be used in a high vacuum environment, in order for thermophoresis to be used in an EUV system to protect a reticle mounted in a reticle chuck, gas at a pressure of approximately fifty milliTorr (mTorr) or more may be introduced to substantially flow around the reticle. With the gas at a pressure of approximately fifty mTorr or more flowing around the reticle, particles may be effectively pushed away from the reticle towards a cooler surface. As will be appreciated by those skilled in the art, at pressures close to zero, thermophoretic forces are relatively insignificant. However, at low pressures of approximately fifty mTorr, thermophoretic forces are generally significant enough to convey particles from a hotter area to a cooler area.
FIG. 1 is a diagrammatic side view representation of a portion of an EUV lithography or exposure system. An EUV lithography system 100 includes a chamber 104 which includes a first region 108 and a second region 110. First region 108 is arranged to house a reticle stage 114 which supports a reticle chuck 118 that holds a reticle 122. Second region 110 is arranged to house projection optics (not shown) and a wafer stage arrangement (not shown). Sections 108, 110 are substantially separated by a differential pumping barrier 126 through which an opening 130 is defined.
Gas at a pressure of around fifty mTorr or more is supplied to first region 108 through a gas supply opening 132 in chamber 104. In order for EUV radiation absorption losses in the gas to be minimized, second region 110 is maintained at a lower pressure, e.g., less than approximately one mTorr, than the pressure maintained in first region 108. Hence, independent differential pumping of first region 108 and second region 110 is maintained by pump 134 and pump 136, respectively, so that the pressure in second region 110 may be maintained at approximately one mTorr or less while gas of a higher pressure is supplied through opening 130 into first region 108.
In order for particles (not shown) located between reticle 122 and barrier 126 to be conveyed away from reticle 122 by the gas using the principles of thermophoresis, a temperature differential must be maintained between reticle 122 and the surroundings of reticle 122. Typically, in order for thermophoresis to convey particles away from reticle 122, reticle 122 is maintained at a higher temperature than barrier 126. When reticle 122 is maintained at a higher temperature than barrier 126, particles (not shown) present between reticle 122 and barrier 126 may be attracted towards barrier 126, as will be discussed below with respect to FIG. 2. In come cases, particles (not shown) that are attracted towards barrier 126 may pass into second region 110 through opening 130. The flow of gas from region 108 to region 110 will also convey particles away from reticle 122, which helps in keeping particles from coming into contact with reticle 122.
With reference to FIG. 2, the use of thermophoresis to substantially repel particles away from the surface of a reticle will be described. A reticle 222, which is maintained at a first temperature, may be positioned in proximity to a cooler surface 226. Cooler surface 226 may be a differential pumping barrier in a chamber used in EUV lithography, or may be a shield which is arranged to protect reticle 222. A variation in gas temperature is generally formed between reticle 222 and cooler surface 226 that goes from being relatively warm near reticle 222 to being relatively cool near cooler surface 226. This creates a temperature gradient in the gas which is an essential condition for the existence of thermophoresis. Particles 228 are generally repelled from reticle 222 towards cooler surface 226. That is, thermophoretic forces are such that particles are driven away from the hotter reticle 222 towards cooler surface 226. Some particles 228 may become substantially attached to cooler surface 226.
While the positioning of a surface in proximity to a reticle that is cooler than the reticle reduces particle contamination of the reticle, maintaining surfaces of different temperatures within an EUV apparatus is often problematic. For example, maintaining surfaces at different temperatures may complicate temperature control of critical systems. In addition, issues relating to thermal expansion and distortion typically arise when a reticle and adjacent components are maintained at different temperatures. When there is thermal expansion or distortion within an EUV apparatus, e.g., with respect to a reticle or a shield, the integrity of an overall lithography process or, more generally, a semiconductor fabrication process may be compromised. Also, the flow of gas from region 108 of chamber 104 to region 110 may sweep particles originating in region 108 into proximity with reticle 122, thereby increasing the risk of contamination despite the protection afforded by thermophoresis.
Therefore, what is desired is a system which allows an EUV reticle to be efficiently and effectively protected from particle contamination substantially without adversely affecting an overall EUV lithography process. That is, what is needed is a system which enables a reticle such as an EUV reticle to be protected from particle contamination without a significant risk of thermal expansion and distortion issues arising.